During the last decade the science of genetic toxicology has experienced dramatic growth in its volume of experimentation, its variety of assays, and its level of public awareness. This growth, in all its dimensions, is attributable to the ability of these short-term tests to detect, rapidly and relatively inexpensively, environmental agents that are genotoxic. These agents are thought to be implicated in such diverse human health problems as cancer, aging and birth defects. Scientific research in genetic toxicology, however, is far from mature; much remains to be achieved in terms of understanding the precise implications of results from such tests for the assessment of risks to human health. To this end, it is important that there be objective methods of analysis of assay system test results, so that subjective assessments do not significantly interfere with the evaluation of the relative merits of short-term tests. Thus, development of appropriate statistical techniques for the analysis of data arising from short-term assays remains the primary goal of this project. In addition to the development of statistical analyses, an increasing amount of attention has been paid to: (i) methods for meaningful assay validation and (ii) the exploitation of large databases for the assessment of interlaboratory and interassay concordance. In response to the heightened interest in in vivo mechanisms of genetic toxicity, this project has been broadened to include study of a number of in vivo tests. Efforts have also been expended to explore those issues that are uniquely characteristic of genetic toxicity studies of human subjects, an area of increasing research activity.